Position sensors that work by using the variable reluctance of an electromagnetic field about an inductor have been known for many years. With these devices relative motion between an actuator and an inductor (coil) causes a change in the reactance of the coil and that change is used to cause an associated circuit to have a change in electrical output. With the prior art the most common circuit has generally consisted of three functional groups. See FIG. 1. Those functional groups are an oscillator, a reactive unit, and an amplifier. Alternating current produced by the oscillator is fed to the reactive unit. The reactive unit consists of coil 2 in parallel with capacitor 4. Actuator 3 moves in relation to coil 2 and causes the reactance of coil 2 to change. Coil 2 and actuator 3 make up variable inductor 1. Voltage drop across the reactive unit is amplified for output. A change in reactance caused by a change in position of the actuator relative to the inductor causes a change in output of the amplifier. In this way the relative position of the inductor and actuator is determined. There have been variations on the arrangement of the above three functional groups, but in most cases the principle used has been that an oscillating current fed to an inductor has a voltage drop across the inductor, and if the voltage drop changes that change can be measured. With the prior art sensor arrangement the inductor 2 is connected directly to ground. Voltage across the inductor equals the resistance of the inductor times the current through the inductor. Since the electrical resistance of variable inductors used for variable reluctance position measuring devices is small and since the current through the reactive unit must be small the voltage drop across the reactive unit must be small. Thus with the prior art the voltage differential of the reactive unit is small. Because the voltage differential is small, the resolution of measurement is small and also effected by electromagnetic noise and stray capacitance in the circuit and environment. The cause of the low voltage differential with prior art variable reluctance sensors is the result of the circuit used and also the physical characteristics of the sensors employed.
Another type of prior art circuit uses two oscillators. One oscillator of fixed frequency and one oscillator that includes the sensor coil in a tank circuit where the coil and a capacitor are wired in parallel. The frequency of the tank circuit changes with change in inductance and the difference between the frequency of the fixed oscillator and the variable oscillator are compared. These circuits add much complexity for little improvement in performance and are seldom used.
The prior art induction sensors can be divided into two types. One type, as in FIG. 6, uses a coil 2 with a space inside of it in which an actuator 3 resides. Prior art sensors of this type suffer from the fact that the output is not linear with regard to position.
Another sensor type of the prior art is the proximity device shown in FIG. 7. Here coil 2 is enclosed in a ferrite housing 1 that covers the outside diameter of the coil and also one end of the coil. A section of ferrite also fills the space inside of the coil and is in contact with the ferrite that encloses one end of the coil. In the art this ferrite shape is called a pot core. By means of the pot core, the electromagnetic field 4 produced by the oscillating current in the coil is projected from the ferrite on the end of the coil that is not enclosed by the ferrite. This type of sensor is used for detecting the position of an object 3 as that object moves in a direction parallel to the axis of the coil, and toward or away from the open end of the ferrite housing. That object is the actuator. This type of device may also be used to simply detect the presence or absence of the actuator in the field.
All of the prior art devices experienced various deficiencies in terms of accuracy, functionality, reliability, and cost.
These and other difficulties experienced with the prior art devices have been obviated in a novel manner by the present invention.
It is one purpose of this invention to provide a sensor circuit that can provide better resolution than the prior art.
It is also a purpose of this invention to provide sensors that when used with the sensor circuit of the present invention provide better resolution than the prior art.
It is also a purpose of this invention to provide sensor systems in which the effect of electromagnetic noise and stray capacitance is diminished.
It is another object of this invention to provide a device of the type show in FIG. 6 that has linear output with regard to position.
It is another object of this invention to provide a sensor of the type show in FIG. 6 that is capable of detecting angular position.
It is another object of this invention to provide a variable reluctance sensor for measuring acceleration
It is another object of this invention to provide a device of the type show in FIG. 7 that detects motion of an actuator across the face of the sensor, in a direction perpendicular to the axis of the coil. This device is suitable for detecting linear position of the actuator and is here called a modified pot core sensor.
It is another object of this invention to provide an arrangement for the use of the modified pot core sensor in conjunction with a rotatable shaft where radial position of the shaft can be detected.
It is a further object of the invention to provide a sensor that is capable of being manufactured of high quality and at a low cost, and which is capable of providing a long and useful life with a minimum of maintenance.
With these and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the claims appended hereto, it being understood that changes in the precise embodiment of the invention herein disclosed may be made within the scope of what is claimed without departing from the spirit of the invention.